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How Plants Remove Greenhouse Gases From The Atmosphere

Every day -- 24 hours a day, 7 days a week -- massive amounts of carbon dioxide (CO₂) drift up from the earth’s surface into the atmosphere. The CO₂ is coming from fossil fuel burned by cars and trucks, industrial plants, electrical power generators, and other activities; humans and animals, which exhale CO₂ into the air with every breath; and the decomposition of soil organic matter through tillage and erosion. In addition, atmospheric CO₂ levels are increasing due to the loss of vegetation globally, primarily rain forests. Vegetation absorbs CO₂, so the loss of vegetation means more CO₂ remains in the atmosphere.

Once in the atmosphere, CO₂ hovers near the earth’s surface and traps heat. That’s why it is known as a “greenhouse gas.” Over the past 100 years, CO₂ levels in the earth’s atmosphere have increased dramatically. This has led to concern about global warming.

So what prevents CO₂ from accumulating to such levels in the atmosphere that it gets out of balance with the oxygen, nitrogen, and other elements? The short answer: plants and oceans.

Plants absorb CO₂ from the atmosphere in the process of photosynthesis, says Gary Paulsen, K-State plant physiologist. This accounts for about 93 percent of the total amount of CO₂ released into the atmosphere every day. And CO₂ also diffuses as a gas into ocean water. This accounts for about 4 percent of the total amount of CO₂ entering the atmosphere. The remaining 3 percent is the net annual increase in atmospheric carbon.

By far, plants and photosynthesis represent the most powerful source of CO₂ removal on earth. If the amount of plant life were to decrease, that could result in a large increase in greenhouse gases in the atmosphere along with a decrease in oxygen levels.

The ability of plants to help keep CO₂ levels down and oxygen levels up in the atmosphere is due to the process of photosynthesis, Paulsen says. Here’s how it works:

1. Sunlight enters the cells on the surface of plant leaves, and chlorophyll within the plant cells captures that light energy.

2. The plant cells use some of that energy to split water within the plant cells into its component parts of oxygen and hydrogen. The oxygen atoms are released into the atmosphere.

3. Carbon dioxide from the atmosphere enters into cells through small openings on the surface of plant leaves. Plant cells use the sun’s captured light energy to combine CO₂ with the hydrogen atoms to form sugars and other organic compounds, and release more oxygen. This process is called carbon fixation, which forms the basis for plants’ ability to take carbon out of the atmosphere and fix it, or store it, in an organic form. Glucose can be converted into starch or cellulose; or it can be combined with nitrogen within plants to form proteins.

Ocean plant life, or phytoplankton, also fix carbon through photosynthesis. But instead of removing CO₂ directly from the atmosphere, phytoplankton remove CO₂ from ocean waters and fix it into organic chemicals through photosynthesis. In fact, 75 percent of all photosynthesis on earth takes place within phytoplankton in the ocean waters.

“Plants, both on land and in the water, may be viewed as carbon sinks since they remove carbon dioxide from the atmosphere and fix it into organic chemicals,” Paulsen says.

For more information, contact: 
Dr. Gary Paulsen  
Department of Agronomy
Kansas State University
Manhattan, KS 66506
(785) 532-6101
gmpaul@ksu.edu

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